This invention relates generally to broadband communications systems, such as hybrid/fiber coaxial (HFC) systems, and more specifically to an apparatus for combining optical signals from a plurality of optical transmitters.
FIG. 1 is a block diagram illustrating an example of one branch of a conventional broadband communications system, such as a two-way hybrid/fiber coaxial (HFC) system, that carries optical and electrical signals. Such a system may be used in, for example, a cable television network; a voice delivery network, such as a telephone system; and a data delivery network to name but a few. The communications system 100 includes headend equipment 105 for generating forward signals (e.g., voice, video, or data signals) that are typically transmitted as optical signals in the forward, or downstream, direction along a first communication medium, such as a fiber optic cable 110. Coupled to the headend equipment 105 are optical nodes 115 that convert the optical signals to radio frequency (RF), or electrical, signals. The electrical signals are further transmitted along a second communication medium, such as coaxial cable 120, and are amplified, as necessary, by one or more distribution amplifiers 125 positioned along the communication medium.
Passive splitter/combiner devices 130 may also be added to the network 100 to split the electrical signals in the forward path, thereby delivering signals to separate portions of the network 100. Taps 135 then further split the forward signals for provision to subscriber equipment 140, such as set-top terminals, computers, telephone handsets, modems, and televisions. It will be appreciated that only one branch connecting the headend equipment 105 with the plurality of subscriber equipment 140 is shown for simplicity; however, there are typically several different fiber links connecting the headend equipment 105 with several additional nodes 115, amplifiers 125, and subscriber equipment 140.
In a two-way system, the subscriber equipment 140 can also generate reverse signals that are transmitted upstream through the reverse path to the headend equipment 105. Such reverse signals may be combined via the splitter/combiner devices 130 along with other reverse signals and then amplified by any one or more of the distribution amplifiers 125. The signals are then converted to optical signals by the optical node 115 before being provided to the headend equipment 105. It will be appreciated that in the electrical, or coaxial cable, portion of the network 100, the forward and reverse path signals are carried on the same coaxial cable 120.
A detailed example of forward and reverse optical paths that are suitable for use in a broadband communications system is shown in FIG. 2. Headend equipment 205 generates and transmits optical signals via optical transmitters 210a-n downstream through their respective fiber links 215a-n. It will be appreciated that there are a plurality of optical transmitters 210a-n transmitting optical signals to a plurality of nodes 220a-n, where each node 220 services a different portion of the system depending upon the system design. Within the nodes 220a-n, an optical receiver 230a-n converts the optical signals to electrical signals for delivery through the coaxial portion of the network. Before transmission, a diplex filter 235a-n isolates the forward electrical signals from the reverse electrical signals and provides the electrical signals to coaxial cable 240a-n for delivery to a plurality of subscriber equipment 245-n. 
In the reverse path, electrical signals emanating from the plurality of subscriber equipment 245a-n are transmitted upstream via the coaxial cable 240a-n to the respective node 220a-n. The diplex filter 235a-n isolates the reverse electrical signals from the forward electrical signals and provides the reverse signals to an optical transmitter 250a-n for converting the electrical signals to optical signals for delivery through the fiber portion of the network. The optical signals are then transmitted further upstream via a reverse optical fiber 255a-n to an optical receiver 260a-n that may also be located within the headend. The optical receiver 260a-n converts the optical signals to electrical signals. Each optical receiver 260a-n then transmits the electrical signals to a passive splitter/combiner 265 for combining the electrical signals in the conventional electrical manner. Those skilled in the art will appreciate that, at this point, the electrical signals is the same as if the electrical signals from subscriber equipment was combined and carried back to the headend via analog means. Additional equipment within the headend then receives the combined electrical signal and, based on the bandwidth allocation scheme, routes portions of the signal to the correct equipment for further processing.
If additional subscribers are added to the network, it may be necessary to add an additional node 220 to service those subscribers. The new node would require separate fiber links for the forward and reverse paths to the headend and a single coaxial path to connect to the additional subscriber equipment. Additionally, if the operator chooses to optimize the network to accommodate an increase in the amount of reverse signals being transmitted by one optical transmitter due to an increase in interactive services with the subscriber equipment, an operator can accomplish this by decreasing the number of subscriber homes that a node 220, or path, services. For example, an operator can reduce an existing path that includes 2000 subscriber homes per node to 500 subscriber homes per node, and add three additional paths each including a node to service that portion of the network. It can easily be understood that increasing the size or optimizing the network requires a significant amount of equipment, fiber, and labor.
At certain times, optical signals may be combined via a passive optical combiner, similar to a passive electrical combiner, as long as the optical transmitters, optical combiner, and optical receiver are restricted to a controlled environment. Those skilled in the art will appreciate that when the optical signal from multiple optical transmitters is combined and applied simultaneously to an optical receiver, intermodulation distortion results. If the differences between these received wavelengths are sufficiently small, the intermodulation distortion produced in the optical receiver will obscure the desired electrical signals, which are, for example, signals from 5 Mega Hertz (MHz) to 42 MHz, at the output of the optical receiver. The optical transmitters, therefore, need to transmit the optical signals at different wavelengths in order for the optical receiver to distinguish between them. In a controlled environment, i.e., controlling the temperature of the optical transmitters, the required different wavelengths of the optical signals can then be strictly maintained to avoid drifting due to temperature.
In most real world applications, however, a controlled environment is difficult to achieve. For example, in a broadband communications system, such as the system shown in FIG. 1, many components are exposed to the environment, such as varying regions and temperatures. Consequently, optical transmitters may begin transmitting optical signals at a particular wavelength, but due to heat that is imposed upon the transmitter, for example, the afternoon sun, the wavelength drifts. When signals from several transmitters are combined and at least one of the wavelengths drift, the intermodulation distortion that is produced in the optical receiver will then obscure the desired signals. Moreover, it is not desirable for an operator to use different lasers (i.e., at different wavelengths) within the plurality of optical transmitters in order to transmit optical signals at substantially different wavelengths due to the cost of installing and maintaining essentially different optical transmitters. Therefore, it will be appreciated that separate reverse fiber paths, or links 255a-n, are typically required because the reverse optical signals cannot be combined like the reverse electrical signals.
Therefore, what is needed are devices and networks that are capable of transmitting and combining reverse optical signals, similar to the combining of reverse electrical signals, without having to employ optical transmitters that transmit signals at differing wavelengths. More specifically, the devices and networks need to be able to combine optical signals that may have similar wavelengths, or allow for wavelength drift, while still allowing the optical receiver to distinguish between the different optical signals. Additionally, the operator would like to use existing equipment, such as optical transmitters, that may already be placed within the network.